1. Field of the Invention
The present invention relates to a two-component developing agent containing a toner and a carrier and the carrier.
2. Description of the Related Art
An electrophotographic technique to which a Carlson process is applied has widely been used in an image forming method using a developing agent. In an image forming apparatus using the electrophotographic technique, image forming is performed in a charging step, an exposing step, a developing step, a transferring step, a fixing step, a cleaning step and the like. In the charging step, a surface of a photoreceptor is uniformly charged. In the exposing step, an electrostatic latent image is formed on the surface of the photoreceptor by exposing the thus-charged photoreceptor. In the developing step, a visible image is formed by allowing a developing agent such as a toner to adhere to the electrostatic latent image formed on the surface of the photoreceptor. In the transferring step, a toner image is transferred onto a recording material by charging the recording material with a polarity opposite to that of the toner. In the fixing step, the visible image transferred onto the recording material is fixed by applying measures such as heat and pressure. In the cleaning step, the toner left over on the surface of the photoreceptor without having been transferred to the recording material is recovered. By performing these steps, an image forming apparatus making use of an electrophotographic process forms a desired image on the recording material. A developing system based on the electrophotographic technique is roughly divided into a one-component developing system and a two-component developing system.
It has heretofore been known that the developing agent to be used in the image forming apparatus making use of the electrophotographic technique contains the toner to be produced by a crushing method, a polymerization method or the like. The crushing method is a method in which a thermoplastic resin, a coloring agent, a charge control agent, a wax as an anti-offset agent and the like are melt-kneaded, cooled to be solidified to prepare a melt-kneaded product and, then, the thus-prepared melt-kneaded product is crushed and classified, to thereby prepare the toner. Further, the polymerization method is a method in which the toner is prepared by a dispersion polymerization method, an emulsion polymerization method or the like.
Presently, a higher speed and a smaller size of the image forming apparatus such as a copying machine or a printer has been attempted. In order to obtain a high-quality image for a long period of time, development of the developing agent excellent in durability and environmental stability has been required.
In the case of a two-component developing agent consisting of a toner and a carrier, in order to obtain an developing agent excellent in durability and environmental stability, research and development of carriers such as optimization of core material type, coating resin type and coating resin amount is important for the purpose of stably charging a developing agent inside an image forming apparatus as well as research and development of toners.
A typical conventional technique for the development of the carrier is described in Japanese Unexamined Patent Publication JP-A 4-177369. A color developing agent as described in the JP-A 4-177369 is a developing agent containing a toner and a carrier, in which, in the carrier, a carrier core material is coated with a predetermined resin (coating resin) in an amount of 0.1 to 5.0% by weight on the basis of the weight of the carrier core material.
Further, another technique is described in Japanese Unexamined Patent Publication JP-A 2003-255591. A two-component developing agent for electrophotography as described in the JP-A 2003-255591 is a developing agent containing a toner and a carrier, in which, in the carrier, a carrier core material is coated with a coating material (coating resin) in an amount of more than 5.0% by weight on the basis of the weight of the carrier core material.
It is necessary that the developing agent has appropriate fluidity and chargeability such that the toner of the developing agent smoothly adheres to an electrostatic latent image formed on a surface of a photoreceptor. The fluidity and chargeability of the developing agent vary in accordance with compositions of the carrier.
According to the color developing agent as described in Japanese Unexamined Patent Publication JP-A 4-177369, by using the carrier in which the carrier core material is coated with the predetermined resin in an amount of 0.1 to 5.0% by weight on the basis of the weight of the carrier core material, a contamination in which the toner forms a thin film on a surface of the carrier or the like is prevented, to thereby prepare the developing agent excellent in the durability and the environmental stability. However, in recent years, a higher speed operation and a smaller size of the image forming apparatus has been realized. At the time of forming an image by using such image forming apparatus as described above, since a stress to be given to the developing agent comes to be larger than that in the conventional image forming apparatus, the resin is peeled off from the core material in use for a long period of time, to thereby expose the surface of the carrier core material. Namely, in a case of forming the image by using the image forming apparatus which has been down-sized and is capable of performing a high speed operation, the developing agent has an insufficient durability incapable of maintaining carrier characteristics of an initial stage and, then, the image to be obtained causes a low image density or the like.
Further, according to the two-component developing agent for electrophotography as described in Japanese Unexamined Patent Publication JP-A 2003-255591, by coating the carrier core material with a coating material in an amount of more than 5.0% by weight, even when the developing agent is used for a long period of time in the image forming apparatus which has been down-sized and is capable of performing a high speed operation, the surface of the carrier core material is unlikely to be exposed and, accordingly, the developing agent comes to be excellent in durability such that it can maintain the carrier characteristics for a long period of time. However, since the carrier core material is coated with a large amount of the coating material, there is a possibility in that, depending on types of coating materials, carrier characteristics such as environmental characteristics can not be maintained and, further, at the time of forming the image, a toner concentration in the developing agent is not stabilized in a high-temperature high-moisture atmosphere and, then, there is also a possibility of causing toner scattering, background fogging and the like. Therefore, magnetite has externally been added to the toner, to thereby solve such problems as described above.
Further, the inventors have found that, even when a coating amount of the carrier is 5.0% by weight or more on the basis of the weight of the core material, by defining the toner concentration by a coating ratio of the toner to the carrier, the developing agent excellent in the durability and the environmental stability can be obtained.
Namely, in such methods as described above, even when the carrier in which the coating amount is 5.0% by weight or more against the core material is used, by defining the toner added with an external additive or the toner concentration, the environmental stability is allowed to be enhanced.